BoilerFaultCodesAlpha 0A18 Fault Code
The Alpha 0A18 fault code means: your boiler has detected an unusual and rapid change in water temperature, causing it to shut down for safety reasons to prevent overheating.. It appears on 3 Alpha models in our database.
Technical description: Flow and return sensor anomaly detected. Sudden temperature change detected on the flow or return sensor.
What causes the Alpha 0A18 fault?
This fault occurs when the boiler's internal sensors detect an impossibly fast rise or fall in temperature, suggesting that water isn't moving through the unit correctly. The most common reason is a circulation failure, often caused by a faulty pump or a blockage in the pipework that prevents heat from being carried away from the heat exchanger.
This issue is more frequent when the heating is first turned on after summer, as pump components can seize up during long periods of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Alpha models that show the 0A18 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Alpha 0A18 fault code information on this page is sourced and cross-checked against the official Alpha service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026